Blood flow is a generic term used to define movement of blood through blood vessels, which can be quantified in terms such as volumetric flow rate (i.e., volume/time) or travel speed (i.e., distance/time). Tissue perfusion is distinguished from vascular blood flow in that tissue perfusion defines movement of blood through blood vessels within a tissue volume. Tissue blood perfusion is often quantified in terms of volume/time/tissue volume, though on occasion tissue mass is used instead of tissue volume. More specifically, tissue perfusion relates to the microcirculatory flow of blood per unit tissue volume in which oxygen and nutrients are provided to, and waste is removed from, the capillary bed of the tissue being perfused. Perfusion is associated with nutritive blood vessels (i.e., micro-vessels known as capillaries) that comprise the vessels associated with exchange of metabolites between blood and tissue, rather than larger diameter non-nutritive vessels. However, compared to blood movement through the larger diameter blood vessels, blood movement through individual capillaries can be highly erratic, principally due to vasomotion, wherein spontaneous oscillation in blood vessel tone manifests as pulsation in erythrocyte movement. In certain cases, for example, vasomotion can result in a temporary arrest of blood flow within the capillary bed for periods of up to 20 seconds, in order to facilitate oxygen diffusion from the individual erythrocytes through the capillary vessel wall and into adjacent tissue being perfused. Consequently, spontaneous oscillations in capillary blood flow can be independent of heart beat, innervation, or respiration. Such flow cannot be defined simply in terms of volume/time; instead, it must be characterized on the basis of the aggregate amount of blood in all the blood vessel (i.e., capillary) segments within a given volume of tissue. This characterization is reflected in the fact that all the measurements of capillary blood movement include a tissue volume-related dimension.
There are many circumstances in which medical practitioners and other clinicians desire to correctly assess blood flow and/or tissue perfusion in tissue. For example, in treating patients with wounded tissue, clinicians must correctly assess blood flow and/or tissue perfusion in and around a wound site, since poor tissue perfusion will have an adverse effect on the healing process. An accurate assessment of blood flow and/or tissue perfusion increases the chances of successful healing of both acute (e.g., surgical) and chronic wounds. The assessment of perfusion dynamics is also important in other clinical applications, such as pre-surgical evaluation of patients undergoing plastic reconstruction procedures (e.g., skin flap transfers), or assessment of viability and function of cardiac tissue during cardiac surgery (e.g., coronary artery bypass graft surgery, a partial left ventriculectomy or a left ventricular reduction via the Batista surgical procedure, etc.).
Presently, clinicians may rely on clinical judgment based on subjective visual assessment and crude mechanical tests to assess tissue perfusion. One such crude mechanical test is a capillary refill test, in which the clinician applies pressure to an external capillary bed (e.g., by pressing on a nail bed) to cause blanching as blood is forced from the tissue, then measures the time needed for color to return after the pressure is released. However, such clinical judgment is subjective and not very precise.
Certain advanced practices have begun to use imaging technologies such as fluorescence imaging technologies for assessing blood flow and/or tissue perfusion. Fluorescence imaging technologies typically employ the administration of a bolus of an imaging agent (such as for example, indocyanine green (ICG), which binds with blood proteins in a subject) that subsequently circulates throughout the subject's vasculature and emits a fluorescence signal when illuminated with the appropriate excitation light. Fluorescence imaging systems acquire images of the emitted imaging agent fluorescence as the imaging agent bolus traverses the subject's tissue in the imaging field of view. The images are typically acquired as the bolus enters the tissue through arterial vessels, travels through the tissue's microvasculature, and exits the tissue through the venous vessels. When the images are displayed as video on a monitor, clinicians may observe this imaging agent transit in the vasculature represented as variations in fluorescence intensity with time. Based on their visual perception of the fluorescence intensity, clinicians may make a relative, qualitative determination regarding the blood flow and/or perfusion status of the tissue and its subsequent healing potential. However, a qualitative visual evaluation of such images is not always sufficient for a number of reasons, particularly in instances where the visual information is ambiguous. For instance, such visual evaluation is limited since many parameters, such as image brightness, image contrast and image noise, can be affected by factors other than the blood flow and/or perfusion properties of the tissue. Moreover, mere visual evaluation is subjective (e.g., visual evaluation may vary from clinician to clinician, one clinician's visual evaluation protocol may vary somewhat from patient to patient and/or from imaging session to imaging session) and does not support a standardized protocol for assessing blood flow and/or tissue perfusion. Finally, due to a clinician's lack of memory or inaccurate recollection of previous visual assessments, it can be challenging to reliably and consistently compare and track blood flow and/or perfusion status of a patient over time across multiple imaging sessions.
Thus, it is desirable to have methods and systems to process and/or present medical image data to the clinician in a manner that characterizes blood flow and/or tissue perfusion in an accurate, convenient, and easily understood fashion.